For the methods currently used for the manufacture of commercial crystalline solar cells, a cost reduction is the important problem. To this end, a combination of heat diffusion process with screen printing process is generally employed. The detail of the method is as illustrated in FIG. 1, for example.
First, a single crystal silicon ingot pulled up by the Czochralski (CZ) method or a polycrystalline silicon ingot prepared by the casting method is sliced by the multi-wire method, yielding a p-type silicon substrate (Step (1)). Next, slice damages are removed from the surface by an alkaline solution, and a texture with a maximum height of about 10 μm is formed on the surface (Step (2)). An n-type diffusion layer is formed in the substrate surface by a heat diffusion process (Step (3)). Further, a silicon nitride film is deposited on a light-receiving surface, typically to a thickness of about 70 nm, forming an antireflection/passivation film. Next, the glass formed on the substrate surface is etched away, and cleaning treatment is carried out (Step (4)), after which an antireflective film is formed on the light-receiving surface side of the substrate (Step (5)). Next, using a screen printing process, an aluminum-based electrode paste is printed over the entire back surface of the substrate which is a non-light-receiving surface, and dried to form a back electrode (Step (6)). Next, on the light-receiving surface side of the substrate, an electrode paste (or electrode agent) containing metal particles such as silver and additives such as glass frit is screen printed in a comb-shaped pattern with a width of about 100 to 200 μm, and dried (Step (7)). Subsequently, junction isolation treatment is carried out (Step (8)), and the overall substrate is heat treated to fire the electrode paste-applied portion into a front electrode (Step (99)). This heat treatment causes to fire metal particles in the electrode paste for suppressing interconnect resistance and glass frit to penetrate through the silicon nitride film (known as fire-through), for thereby providing conduction between the light-receiving surface electrode and the diffusion layer, and forming an electric field layer of Al—Si at the interface between the non-light-receiving surface electrode and the silicon substrate.
With regard to the electrode firing heat treatment, for example, JP-A 2011-258813 (Patent Document 1) describes that for the electrode firing heat treatment, the heating zone is typically at a temperature of 500 to 950° C., especially 600 to 850° C., preferably for a heating time of 5 to 30 seconds, and the cooling zone is at a temperature of 25 to 500° C., preferably for a cooling time of 5 to 30 seconds. The heating temperature includes a relatively high temperature range.
However, in order to form an electrode with long-term reliability through the above-mentioned electrode firing heat treatment, the peak temperature of the electrode firing heat treatment must be 800° C. or higher for the purpose of promoting firing of silver particles. At this point, the substrate is also exposed to high temperature so that the bulk lifetime of the substrate is reduced, and the surface recombination velocity is increased, giving rise to the problem of failing to maintain high conversion efficiency.
Notably, JP-A 2012-514342 (Patent Document 2) is one of previous documents relevant to the present invention.